The following publications are representative of the most relevant prior art known to the Applicants at the time of filing the application.
______________________________________ U.S. PAT. NOS. ______________________________________ 3,481,780 December 2, 1969 S. P. Mitoff 3,492,153 January 27, 1970 G. Ervin, Jr. 3,509,072 April 28, 1970 J. Barrington et al 3,875,476 April 1, 1975 W. B. Crandell et al 3,875,477 April 1, 1975 J. I. Fredriksson et al 4,120,829 October 12, 1978 F. H. Dulin 4,187,344 February 5, 1980 J. I. Fredriksson ______________________________________
Porous refractory bricks, kiln furniture, resistor bars and the like formed of sintered refractory grains are subjected to oxidizing atmospheres in their normal usage and are known to oxidize producing stresses which cause them to crack, warp or become brittle such that their utility is decreased or destroyed. It is also known that sintered and bonded e.g. silicon nitride bonded silicon carbide electrical elements such as heating elements and igniters are particularly susceptible to oxidative deterioration, the severity of which depends upon the usage to which the particular element is put.
Sintered silicon carbide igniter elements positioned near a gas nozzle have contact with the gas flame and are greatly affected by this corrosive environment. The igniter oxidation problem is particularly acute in the ovens of kitchen gas stoves where the manufacturer's specifications are very restrictive. When installed in the oven of a gas stove, the igniter is energized with an electric current. After a prescribed amperage flow is reached the gas flow controls are activated to open the gas valve permitting a flow of gas that is ignited to heat the oven. The igniter remains energized as long as the oven is in use even though the gas flame is turned off and on again in response to the thermostat controlling the oven's temperature. The oven igniter which sits above or beside the gas burner, is bathed in the intermittently ignited gas flame until the cooking cycle has been completed. Such placement of the igniter, coupled with the strict limitation on amperage variation permitted within the usual specifications written for this usage, represents a severe environment which causes oxidative deterioration of the silicon carbide. Therefore it is desirable to protect the silicon carbide from oxidation.
An igniter to which this invention may be applied is shown in the U.S. Pat. No. 3,875,477 to Fredriksson and Coes, Apr. 1, 1975. The structure there described is a sintered silicon carbide body that has found widespread use in gas and liquid fuel burning systems such as clothes dryers, furnaces, etc., but has suffered a somewhat shortened life expectancy when used in the more severe conditions present in ovens of gas stoves as discussed above.
One proposal for minimizing the destructive effects of oxidation on refractory bodies is described in U.S. Pat. No. 4,187,344 to Fredriksson, Feb. 5, 1980. Silicon nitride and/or silicon oxynitride particles, suspended in a liquid slurry are carried into the pores of the refractory article to provide a protective surface coating. The particles are carried into the body of the refractory, beyond the entrance to the exposed surface pores and after the slurry has dried and the article is fired at 1000.degree. C. to fix the silicon oxynitride or silicon nitride particles in place. The protective layer or coating made with these nitride particles is more or less permanently embedded in place to provide a degree of protection in the form of a barrier against undue oxidation of the igniter in certain hostile environments. However, it has been found in practice, that when these igniters are operated in the gas flame within the oven of a stove, that the water vapor present along with the other products of combustion of the gas detrimentally react with the sub-micron sized silicon nitride or silicon oxynitride impregnant particles. An igniter filled with either of these nitrides, while showing some improvement in retardation of oxidative aging in this environment, was not as long lived as was needed for complete satisfaction in the gas oven field.
In other approaches to a similar problem refractory silicon carbide bodies used for rocket nozzles and high temperature heating elements have been densified as described in U.S. Pat. No. 3,492,153 to Ervin Jan. 27, 1970 by forming aluminum nitride in situ within the pores of the body by reacting aluminum vapor with nitrogen gas within the pores of the silicon carbide article. The aluminum nitride thus formed deposits from the vapor state onto the walls of the pores of the body. The patent also suggests that ammonia and aluminum trichloride or aluminum iodide and nitrogen can be reacted to produce an aluminum nitride vapor that will under the conditions described in the patent, deposit as a solid within the pores of such a silicon carbide article. This structure does not have an acceptable life in a gas oven environment because of the reactivity between the water vapor in the gas flame and the aluminum nitride impregnant.
The Mitoff reference discloses the densification of coatings of zirconia and thoria by painting these coatings with a concentrated aqueous solution of iron chloride, usually several applications of the solution are required, heating the painted body to a temperature in excess 1300.degree. C. in a gaseous reducing atmosphere having a controlled oxygen partial pressure such that essentially only ferrous oxide is produced in the pores of the zirconia and thoria coatings.
Attempts to make igniters more resistant to oxidative deterioration are shown in U.S. Pat. No. 3,509,072 to Barrington et al. Apr. 28, 1970; U.S. Pat. No. 3,875,476 to Crandall et al. Apr. 1, 1975; U.S. Pat. No. 4,120,829 to Dulin Oct. 17, 1978 and U.S. Pat. No. 4,204,863 to Schreiner May 27, 1980. All of these disclosures describe the use of various bonding compositions from which an igniter may be constructed in a manner to improve its serviceability. However, in following each of these inventions described in the respective patents the electrical characteristics of the resulting igniter are materially altered.